Apparatus for drawing sheet glass



D. E. SHARP 470,526

APPARATUS FOR DRAWING` SHEET GLASS May 17, 1949.

3 Sheets-Sheet l 'Filed Deo. 6, 1944 :inventor DONALD E //fK/Pi @d-Mmmm May 17, 1949. D. E. SHARP 2,470,526

APPARATUS FOR DRAWING SHEET GLASS Filed Dec. e, 1944 s sheets-sheet 2' Zmventor May W, 1949. D. E. SHARP 2,470,526

APPARATUsfFoR DRAWING SHEET GLASS Filed Dec. 6, 1944 s sheets-sheet 3 INVENTOR. DONALD E. SHA/QP imag A ATTORNE S Patented May 17, 1949 UNITED STATES The present invention relates to apparatus formation i'sls'ubliected' t *different temperature `for producing-sheetV glass, .andv particularly flat conditionsicaii'siig thick a'd thin streaks in the wglass whoselsurfaceshave a natural Vlire nish. glass 'sheeti'tself1""Inarcaswhere cold air 'strikes Such flat glass is generallyrreferred to in the v the unset glass, 'ittv'ill'chili oi'"setmorerapidly,

\:trade asfsheet-glass orlwindow glass, distin- 5 and thus be 'thickerin "the finished sheet,` than `guishing itlfromfplate glass Whose surfaces are adjacent areas,i'eiposed`:tol greater temperatures mechanicallygrlound and polished. Throughout so th'a't th'eslieet willntinuet'attenuate and this application .andinithe claims, the expression becomeir'elatively thinner-"before setting.' vOutsheet glass. will be used in this sense, sideairigairiingentranceat this pintmay also The-various machines-employed today in the l ,introduce 'dirt andl'othr foreign ln'iatterWhich Vproduction ofA sheetglass all have the common may embed its'e'lfiii the glass causingpermanent i Acharacteristics'-.that thesheet is drawn in fiat y defects, or may lodge?uponfollsvvith/which the i#form'continuouslyfromfa'bath supplied by a tank sheet comes in'fcont'act causing scratching', digs, i furnace.- l.The glass-makingingredients are netc. l

troduced into i the; tank furnace, and are .then 15 l Ihave devisedmeaisifor establishingpredeter- 1 .progressively melted\,re1'ined and conditioned for mined and 'controlled air `conditions in-theA zone i proper Withdrawall in sheet form from the Workefshe'et formation; 4.lBasicall-y, objectionable and .ing zend,A y'Thethickness of. the sheet drawn is uncontrolled-movements' -f air of -varying.-tem controlled by balancing 'the-rate of kwithdrawal of l peratures .are previerited'l orf-olst -by -positively i the -sheet from its `source with they Workingv tem- 20 creating movements of air from approximately the perature of the.moltenfglass itself. center of the sheet toward tbothV sides thereof.

a Technological@developments in'glass batches, Theairf-'usedionmovd freni .the center of the 1- ceramics, :glassrforming-wmachinery and techsheet toward its lsides isi-intended to bethat air niques, have made it possible to prepare in a .already presenti-above.theveglass-or,fstated-dif- `:;5i.11g1e tank ylarge .tonnages of glass suitable for 25 ferently, th ail? otaiied'within-the apparatus. idravvinginto asheet oficonsiderable Width. The A The expression '.air islusd nbroadsense, inl"present invention is concernedprimarily with the cluding airfgasesi--products of combustion, etc. L :control offconditi'ons in the-area or zone of sheet The volume Vand"velocity" ofthe air-currents are :formation Assumingthemolten glass is properly i controlled vtoffset or -overbalance the A.tendency `fprepared.-zancliconditinned, that is, ythe batch inof iniiltration ofoutsidelco1d air and also to-cause -gredients havebeenfmeted and refined te'give a a definite movement of warm.. air. from .the-heart metal. qualityisubstantiallyf devoid ofA inherent of .the she`et-towad thenorir'ially. coolerledges, f'. vmetal defects, there is the' further problem of con- .which dmovement is counter to -thefnormaltenverting the molten glass to sheet form having a f dency of the-"air iSOmOierOm-thetelden-edges .thicknesswhichrisiadequately: uniform from edge ofthe sheettoward its center.

toedge', and the surfaces ofwhich are relatively Thus,- the mainf 'objectv of .the invention is to `free from, imperfections, advance th'e technique of drawing sheet glass The condition ofntheiatmosphere surrounding toward the end of producing-asheet having im- `the sheetfdurin'g itsvperiod of formation has a provedvthicknessy andlsurface.fcndition"characsubstantialinuence on its uniformity of thick- 40 telisics.

ness .andsurfacesrIThe temperature of the at l In the drawings wherein like .numeralsare emi. mosphereabover-the'mass of molten glass from i DiOyed '60 designate like parts throughout the which thersheet is withdrawn is relatively high i Ysame:

but is notvuniformacross the sheet or its base. Fig. 1 is a..fragrn-entaryl-vrticfil-llgiilldirlal `Even in those-cases Wherethe good practice of 45 Sectiolillustrati'gf diagiamlaiially 'Oneiype 0f enclosing the .rilasci-line is followed, there is a consheet glaS's drawing machine;

i siderable .infiltration of youtside, relatively cold 2 iS atransvers vrticalsection ofthe same air. Generallyspeaking,this airis drawn in from e `typt 0f lilhin SHOW iih simplest fOrH 0f my the'edges of thessheet, passingupwardly toward air flow control device; 1 y the center;rising-continually as it heats up and Figli isafragmentary planview of a form of expands.v 'These-movements of air and conveck.drivemechansxirforapair ofthe air-owcontrol tion: currents..rlsult` i-nuncontrolled turbulence, Vdevicesand,supportingmeans therefor; y l contributing. ...heavily to airetemperatures which -..Fig. 4 isla Ydetail illustrating avvater-cooled are notuniform throughout or constant, in con- Shield orlaaflie.` used in-conjunction.with-an air sequence of .which-.the-glass initsf=zone of sheet impeller;

Fig. is an enlarged detailed View indicating a modification of the air impeller and shield arrangement;

Fig. 6 is a sectional detail of a modification wherein two air impellers are associated with a single shield or baie;

Fig. 7 diagrammatically shows a pair of individual air flow devices on both sides of the sheet;

Fig. 8 is a detail of a paddle wheel type of air impeller;

Fig. 9 is a fragmentary elevation of the impeller shown in Fig. 8;

Fig. 10 is a fragmentary side view of one end of the water cooled shield or baille of Fig. 4, showing the mounting and adjusting means therefor;

Fig. 11 is a View taken substantially on the line II--II in Fig. 10;

Fig. 12 is a fragmentary plan view of one end of the curved shields of Fig. 5, showing the temperature regulating controls therefor; and

Fig. 13 is a fragmentary side view of the shield of Fig. 6, showing the air conduits leading to and from the air passages therein.

My general type of apparatus can be used in conjunction with any of the well known sheet glass forming machines. The particular machine illustrated in the drawings is known as the Colburn type, and it will be appreciated that the same principles are involved for controlling the air in the area of sheet formation of the other types of machines.

In Fig. 1 the numeral Ill designates the working end of a continuous tank furnace, the receptacle at this end of the furnace in the Colburn machine usually being called a drawpot. It is supported on stools II so arranged that the pot can be heated by gas burners (not shown) and contains a mass of molten glass I2 furnished continuously from the tank designated generally by the numeral I3.

A continuously drawn sheet I4 is pulled vertically from the mass of glass I2. The Colburn apparatus is distinguished in part by the fact that while the glass is drawn initially in a vertical plane, it is passed over an internally cooled bending roller I5 so that the sheet may then be carried horizontally through flattening and annealing chambers. The sheet or ribbon I4 in this instance is not cut into individual sheets until after it leaves the annealing leer.

In other types of sheet glass machines, the sheet is carried through vertically disposed leers, and in still another form the sheet is carried upwardly a, substantially greater distance than in the Colburn machine, then cut and transferred as individual sheets to a horizontally disposed leer.

When drawing the glass from the drawpot of a Colburn machine, a meniscus or enlarged base portion I6 is created, as is clearly illustrated in Fig. 5, and the nal sheet thickness is reached some distance above the surface of the bath. To hold the sheet to width, driven, internally cooled knurled edge devices I'I (Figs. 1 and 2) are mounted to operate in the border portions of the meniscus and have a peripheral speed which is somewhat less than that of the bending roller I5 and consequently the sheet itself. Likewise, the standard Colburn machine includes the liptiles I8 and I9 and liptile coolers 20, positioned to protect the rising sheet I4 from excessive blasts of heated air and gases from the furnace and from the pot chamber. The coolers also serve to reduce the temperature of the surface of the glass moving thereunder just before it is pulled into the sheet. All things being equal, the faster the machine is operated, the thinner will be the sheet; and vice-versa, to obtain thicker sheets, the machine is operated at the slower speeds. The speed of draw coupled with the viscosity of the glass determines the average thickness of sheet drawn.

That area disposed generally between the liptile coolers and the bending roll I5 is referred to herein as the zone of sheet formation. Within this Zone is the exposed surface of the glass in the drawpot, the meniscus, and the rising sheet. The distance between the bending roll and the surface of the molten glass is such that the sheet may be brought into contact with the bending roller without marring its surface, provided of course the surface of the roll and the sheet in contact therewith are reasonably free from grit-like foreign matter.

The knurled width maintaining rollers II chill the glass passing therebetween and produce the impressions indicated generally by the numeral 2| in Fig. 2, but this part of the sheet is removed at the exit end of the leer.

There is normally a stack effect through the sheet forming zone, with the predominantly hotter gases rising centrally of the sheet, and this condition is aggravated when outside air is permitted to flow in from the edges of the sheet and move toward its center. These varying temperature conditions, convection currents and other uncontrolled movements of air have an effect upon the sheet during its formative period. Infiltration of outside air, which is bound to occur even when machine enclosures are used, accentuates the non-uniform character of the air conditions present within the machine, and experience has shown that these air movements, producing temperature differentials across the width of the sheet, result in a sheet having non-uniform thickness characteristics. As an additive, these uncontrolled air disturbances, particularly when laden with dirt, have an adverse effect on the surface conditions of the sheet.

I have determined that it is benecial to create positive and controlled movements of air and to propel them continuously from the center of the sheet toward both sides thereof. By preference, the air caused to move in accordance with this principle is that air already contained within the apparatus, and if any supplementary air is used that it be conditioned as to temperature and to remove foreign matter which could injure the surfaces of the sheet.

In Fig. 2 is shown a simple device which can be used to create the desired air movements. A shaft 22, having a bore through which a temperature controlling medium may be circulated, is adapted to be mounted transversely of the drawpot and generally parallel with the sheet I4. Two spirally wound blades are carried by the shaft. In Fig. 2 one blade or n is indicated by the letter A and the other by the letter B. They produce, in effect, a double air screw having right and left hand portions meeting at C approximately midway of the glass sheet. In Fig. 3 is illustrated means for mounting, rotating and cooling a pair of such air screw devices, one on each side of the sheet. Shafts 22 extend through suitable journals or bearings 23 mounted on the supports 24 and 25, and each carries a gear 26, both of which mesh and are driven by the means 21 causing the rolls to rotate toward or away'from one another, depending upon the direction of the drive 21. The elbows 28 indicate connections to annoyance:

a cooling,medium.;supply.'.such;asiwaterrrproer' vided for passage through the shafts tofpreyentr.: theirl overheating. The eiectivenesssof.thegshaft as a cooler for the atmosphere andthe-glasssheet;v will be dependent/ upon thegsize '.of ;the: pipenthe: rate ofilow of the coolant and.the:proxirnityrof. its Dlocation V.with-respect Vtothefsheetl; .'f x

Upon rotation of the shaftalthezscrew blades.: will'inducei; a ow `of i air` which: is. f 'carriedffrom :'r the1center of the .sheettowarditssidese andzit fis e. intended ythat a variable; ,Speedxdriva @suchriasis the variable speedmotor.withgbultein speedfgereeI ducer shown at 55,beyprovidedsothatrthexmove-fment of air can be readily controlledeasazto velocity. Preferably, the airlleayingthelends,of the screw blades is positively withdr-axrznfandfcari.- ried awayfrom .the machine, -as1for exam-plafhyf.; exhausting into ybell-shaped :chambers `29 f.sur.. rounding A the -screwshaft andspacedtherefrom. In rother words,-the internaLbore of.the,:tube-Magog. has a diameter greaterthanthe outside-.diameter of the screwimpeller` shaft .22.' This provides an exhaustingpassageway between the two, and`vl positive withdrawing meansycan be .arrangedfor by connecting the tube 30 toan exhaustingmanifold 3l and pump (not shown).

In. lig.` 4 is shown `a slight modification sin. which a curved housing 321s associated with rotary air propeller 33which may `besimilar to theair screw device Vshown inFig. 2. 'I`he;h0l.1.S-..,V ing or shield 32 is formed from any,suitablenonfr corrosive metal and hasan internal..cl'iamherv through which a `cooling fluid 34 .n'lay,be.-circu. lated. The cooler member is adjustably'mounted by means so, that it can be positioned with relaf.. tion to the rotary air` propeller and,-like'wise, so. the distance between the ends 35`thereof,and the sheet 3@ can be Varied. Suitable mounting and adjusting means for this purposeare illus` trated iii-Fig. l0` and 11; As showntheratlre, shield t2 is providedwith-a hollow; supporting; stub shaft 55' at either end thereof, *which* stub' shafts are rotatably mounted in bearings 51"*an'd are provided with handr wheels rkeyedth-ereto:n Ihe'bearings 5I may beformed'wth dove'tail`Vy slots 5s in their bottomsswhichare slidalcly''cree` ceived on similarly shapedvprojel'stionstliA on thev top surface of blocks 6l carriedonvloraokets 62@ which may be boltedl tothe machinefenclosure' wall. With this construction; -the` hollow -stuby shafts 5t surround :and are=normally=in axial alignment ywith the shaft of the-rotary-air-'impeller` 33.. However, by'looseningr-the hnurledf.A head t3 on the bolt 54 the bearing Elfmay/be slid back, and forth on the block- 6! lto adjustffthe'- cooler member 32 with relation to the air pro. peller, within the limits of the slot 552I -l In orden to vary the distance between-the;ends135"of vthe f shield and the glass sheet being drawnit is 'onlylnecessaryy toloosen the adjusting screwltinthe bearing .5l and to then rotate .the :shield '.by meansl or the hand wheel 58'."

In Fig. .4 only onel rotary air device-is shcwvm..` and iny some cases a device onone sideof-.the2 sheet only maybeV adequate,y but I preferito :pro-z. vide" at least one on each side of thezsheettfor.: fiexibility in insuring properiand adequate coni-w trol of the atmospheresurrounding. thexsheet during Vits .period of formation.v

In Fig. 5 a pair of rotaryraincontrol-membersrffm 3l :is providedand leach has a housinszfdier-fing vin construction lfrom :the -shieldf32iA in-,lig.v` fl. Each cooler 38has a compartment throughlwhieh; Lf.

a cooling medium, 39 .iswpassed .ande-anulan.,- chamber 40 from which air-'fy .ma-y.; beidischarged@` :75t

throughs openingszorl accontnuous. .-slot :4 i i tofbe 11 pickeclafup.; by `fvthglair screw :31:5 and,I ,moredintolr contact :with the glass sheet;`

Asfaprevouslyqmentioned,I normalygfprefer to 'i usefthat.airycontainedewithin:the machine; but for examplef..throughzthe .shield y38',`n`and it is.A ad.-

supplementary z fairer' can bel introduced,

vantageous to'v condition-.the 4airasto tempera ture:andreedomgfrom ioreign. matter before it ,vis `discharged into .the zonel of sheet formation. Obviously;;the. temperature-and pressure of ther air.discharging,.through th'e:openings Illcan beadjusted tof give the desired delicate temperature controlwhen. such airzis mixedland blended witlcli.` theotherfair ioy'thexainscrewsvBl. rIhe curved shields:...38.i'areA .provided .with independent `temperature 'regulating controls, such .as valvesV S1. i in the air line 68, and valves S9,;;l;."ll` andIE/iV inpthepwaterxlines.,13 and 'M (Fig; '12); as fre- Kquently `the extent ofycooling and .amount .of ain. required:;wi1l.not be .identical on lccthy sides of the, sheet.

For-greater iexiloility,two` ornmol-e. air flow inducing devicesmay ,be -used oneach side of. the sheety and vtwo arrangements-of this character.` are;illustrated-in;1igs.x6 and 7. In lFigi-6 a pair of air propelling devices 42 isassociated with'a. single shield 43. Preferably, in operationo this unit, themembers 42 are rotated in the direction ,of the arrows which; induces a now of airinto the openingsjor slot M1-.of the shield communicating with kthepassageway 45. All of this air oanbe conduits; 215, 16,11, 'ltand the openings or slots -.;46.or-may -bet-partially or completely exhausted f exteriorlyof the-machine through conduit 'i9 and cooler. air brought in through conduits Sii, Tl, 18. and .thechannels 46 and fedl through the orices i 46.- As shown in Fig. v13,v conduitsll and 19-.

terminateina ,booster air ,pump and associated three.,way valve,. which facilitate movement ofv airthrough-the conduits in thedesired direction.

The flowsof air into and out of the housing 43 .are indicated .bythe arrows.'v The' shields-.43 are internally cooled by the mediumydl. In addition to theair forced into the shieldcavityv 45. by the screw.- devices n.112supplementary air at lany desired temperature `Acan be added by-way of con.-A duitsllll` and 82 which, together with the action of ,the .coolant `lll, aiords great exibility in the.v control.of.th.eain in the zone of sheet formation v as to quantities, movements and temperatures.

'Ihe 'air propellers arranged on both sides ofr the sheet in Fig. '7 are indicated as spaced `farther apartvfrom one another than the.v rollsllZ Vjust described and without the shields or ,bailles,but,.

they can ofcourse be used.

Stilljanother. modification is indicated in Figs. 8 and 9 wherein a plurality ofspaced vanes `18 are-carried'by the periphery of the spiral fan blade 49 mounted on the internally cooled shaft 50: Preferably,vall of'thevanes 48 are arranged sothatthey are-parallel with the shaft of the impeller.- and tof giveimaximum positive vfeeding ofthe air from the center -of the sheet towardits edges, and further to produce what may be termed a paddle wheel action which drives -the airagainst: ther sheet surface.

In a broad sense, the .various formsrof air vcontrol/devices maybe referred toas rotary coolers, but it will beunderstooodthat I'V do not wishcto imply that, in all instances-the members are con-4 trolled .tomaterially reduce the temperature of'A the.air. SomeV cooling ofy the shafts is required tmpreyent. .overheatingottthe metal parts; .audit` l consider it good practice to design and operate the devices in such a way that they will not sag or warp due to overheating. On the other hand, the air screws having internally cooled supporting shafts, can be operated at such speeds as to move a sucient volume of air that the temperature of the air is not measurably reduced.

Clearly, positive cooling action is also contemplated, and the cooled housings, when associated with the air screws, are in part for the purpose of lending control to the air movements and to reduce to the desired extent the temperature of the air before it is forced into contact with the sheet. In addition to treatment of the contained air, supplementary air of any desired temperature can be blended with the heated air to give the necessary control required for quality glass production.

It is fundamental with my invention that air is positively and continuously moved from the center of the rising sheet toward its edges, which is counter to the normal flow of air in the Zone of sheet formation when my rotary air impellers are not in use. The pitch and size of the spiral blades can be varied from that shown. Further, the air screws are so mounted that they can be taken out and replaced with different sized units as required.

In ordinary usage, and particularly with the Colburn sheet glass machine, the rotary air propellers are operated to move continuous blankets of air in a direction generally downward with respect to the rising sheet, and from the center toward the sides thereof. However, the direction of either one or both can be reversed to move the blanket of air upwardly. Although these positive means create a movement of air, undesirable turbulence and uncontrolled movements are avoided. Temperature conditions affecting the sheet during its period of formation are controlled to avoid spotty and irregular cooling of the glass and consequently cold streaks and hot streaks which, if permitted, give a finished sheet having variations in thickness resulting in an appearance of waviness.

As previously mentioned, the rate at which any given thickness of sheet can be pulled from the molten glass bath is dependent upon the viscosity of the glass. my invention is that the glass can be cooled faster than normal, in the zone of sheet formation, and uniformly so, resulting in an accelerated speed of sheet drawing. Production can be further increased by drawing wider sheets, made possible because the rotary air propellers and temperature controlling means will give the needed conditioning treatment to the glass for uniformity of thickness and acceptable surface characteristics.

It is to be understood that the form of the nvention herewith shown and described is to be taken as the preferred embodiment of the same, and that various changes in the shape, size and arrangement of parts may be resorted to without departing from the spirit of the invention or the scope of the subjoined claims.

I claim:

l. Apparatus for producing sheet glass comprising a receptacle adapted to be continuously supplied with a pool of molten glass, means for drawing a sheet upwardly from said pool, and rotary air flow devices positioned transversely of the sheet in its zone of formation, said devices being positively driven and having pitched blades for inducing a ilow of air laterally of the sheet in An important feature of both directions from the center thereof toward both edges.

2. Apparatus for producing sheet glass comprising a receptacle adapted to be continuously supplied with a pool of molten glass, means for drawing a sheet upwardly from said pool, rotary air flow devices positioned transversely of the sheet in its zone of formation, said devices being positively driven and having means for inducing a iioW of air laterally of the sheet in both directions from the center thereof toward both edges, and means located in alignment with said devices near the edges of the sheet for exhausting the air therefrom.

3. Apparatus for producing sheet glass comprising a receptacle adapted to be continuously supplied with a pool of molten glass, means for drawing a sheet upwardly from said pool, rotary air flow devices positioned transversely of the sheet in its zone of formation, said devices being positively driven and having means for inducing a flow of air laterally of the sheet in both directions from the center thereof toward both edges, and an internally cooled housing means partially surrounding said rotary devices.

4. Apparatus for producing sheet glass comprising a receptacle adapted to be continuously supplied with a pool of molten glass, means for drawing a sheet upwardly from said pool, a pair of rotary air feed devices mounted above said pool and one positioned on each side of the sheet adjacent thereto, each device having a right and left feeding section meeting approximately at the center of the sheet, and means for rotating said devices at a speed sufficient to positively move air from the sheet center toward both sides thereof.

5. Apparatus for producing sheet glass comprising a receptacle adapted to be continuously supplied with a pool of molten glass, means for drawing a sheet upwardly from said pool, a pair of rotary air feed devices mounted above said pool and one positioned on each side of the sheet adjacent thereto, each device having a right and left feeding section meeting approximately at the center of the sheet, means for rotating said devices at a speed sufficient to positively move air from the sheet center toward both sides thereof, and water-cooled members associated with each of the rotary devices for reducing the temperature of the air moved thereby.

6. Apparatus for producing sheet glass comprising a receptacle adapted to be continuously supplied with a pool of molten glass, means for drawing a sheet upwardly from said pool, a pair of rotary air feed devices mounted above said pool and one positioned on each side of the sheet adjacent thereto, each device having a right and left feeding section meeting approximately at the center of the sheet, means for rotating said devices at a speed sufficient to positively move air from the sheet center toward both sides thereof, and water-cooled members associated with each of the rotary devices for reducing the temperature of the air moved thereby, the water-cooled members having air passages and outlets through which air is fed to the rotary devices.

7. Apparatus for producing sheet glass comprising a receptacle adapted to be continuously supplied with a pool of molten glass, means for drawing a sheet upwardly from said pool, a pair of rotary air feed devices mounted above said pool and one positioned on each side of the sheet adjacent thereto, each device having a right and left feeding section meeting approximately at the center of the sheet, means for rotating said devices at a speed sufficient to positively move air from the sheet center toward both sides thereof, water-cooled members associated with each of the rotary devices for reducing the temperature of the air moved thereby, the water-cooled members having air passages and outlets through which air is fed to the rotary devices, and means for exhausting the air from the ends of said rotary devices.

8. Apparatus for producing sheet glass comprising a receptacle adapted to be continuously supplied with a pool of molten glass, means for drawing a sheet upwardly from said pool, a rotary air flow device positioned transversely of the sheet in its zone of sheet formation, said device being positively driven and having pitched blades for inducing a iioW of air laterally of the sheet in both directions from the center thereof toward both edges, and a cooling member partially surrounding said rotary air flow device.

9. Apparatus for producing sheet glass com- 10 prising a receptacle adapted to be continuously supplied with a pool of molten glass, means for drawing a sheet upwardly from said pool, and a rotary air iiow device positioned transversely of the sheet in its zone of sheet formation, said device comprising a shaft, a pitched blade on said shaft, vanes carried by said blade and in angular relation thereto, and means for driving said shaft.

DONALD E. SHARP.

REFERENCES CETED The following references are of record in the file of this patent:

UNTED STATES PATENTS Number Name Date 1,320,091 Proeger Oct. 28, 1919 2,158,669 Amsler May 16, 1939 2,287,136 Rolland et al June 23, 1942 20 2,352,539 Haibach et a1 June 27, 1944 

